“A book is a gift you can open again and again.” –Garrison Keillor–

Worldwide temperature measurements, corrected for instrumental and measurement errors that result from such factors as changing instruments or urbanization, indicate that global mean annual surface temperatures have increased about 0.3 to 0.6°C during the last 150 years (Nicholls et al., 1996). Since 1975, the increase of the 5-year mean temperature is about 0.5°C – a rate that is faster than for any previous period of equal length in the instrumental record (NASA, 1999). Global surface temperatures in 1998 were the warmest since reliable instrumental measurements began, and seven of the ten warmest years occurred in the 1990s. Temperature reconstructions using proxy climate records such as tree rings, fossil pollen, corals and ice cores and borehole measurements indicate that the 20th century warming is unusual relative to the last 400 to 600 years (e.g., Mann et al., 1998, Overpeck et al., 1997; Pollack et al., 1998).

The observed magnitude and patterns of temperature change during the last century correspond well with model predictions when greenhouse gas, solar, and aerosol forcings are considered together (Wigley, 1999). Most of the recent warming has been in winter over the high mid-latitudes of the Northern Hemisphere continents, between 40 and 70° N (Nicholls et al., 1996). There has also been a general trend toward reduced diurnal temperature range, mostly because nights have warmed more than days. Since 1950, minimum temperatures on land have increased about twice as fast as maximum temperatures (Easterling et al., 1997). This may be attributable in part to increasing cloudiness, which reduces daytime warming by reflection of sunlight and retards the nighttime loss of heat (Karl et al., 1997). Increased soil moisture also reduces daytime warming because part of the solar energy is used to evaporate the excess moisture.